1. Field of the Invention
The present invention relates to an optical recording medium having three or more information recording layers and a method for manufacturing the optical recording medium.
2. Description of the Related Art
Conventionally, optical recording media such as CD-DA, CD-ROM, CD-R, CD-RW, DVD-ROM, DVD-R, DVD+/−RW, DVD-RAM, and Blu-ray Disc (BD) have been extensively used to view digital moving image contents or to record digital data. Among these media, the BD regarded as one of the next-generation DVD standards is configured such that a laser beam of a wavelength as short as 405 nm is used for read and write operations, with the numerical aperture of the objective lens set at 0.85. The optical recording medium compatible with the BD standards has tracks formed at intervals of 0.1 to 0.5 μm. These specifications make it possible to read and write data of 25 GB or greater on one information recording layer of the optical recording medium.
As a future trend, the capacity of moving images or data is expected to grow increasingly from now on. Accordingly, intensive studies have been made on methods for increasing the capacity of the optical recording medium by providing the medium with multiple information recording layers. For the optical recording medium compatible with the BD standards, a new technique has been reported which realizes an ultrahigh capacity of 200 GB by providing six to eight information recording layers.    [Non-Patent Document 1] I. Ichimura et. al., Appl. Opt, 45, 1974-1803 (2006)    [Non-Patent Document 2] K. Mishima et. al., Proc. of SPIE, 6282 and 628201 (2006)
In general, the multi-layered optical recording medium has a problem that during read and write operations on a target information recording layer thereof, the target layer may be affected by leakage of signals or noise (crosstalk) from another information recording layer, resulting in the servo signal or recorded signal being degraded in quality.
On the other hand, as the current BD-type optical recording medium, a multi-layered optical recording medium with two information recording layers has been put into practical use. This dual-layer structure makes it possible to provide a sufficient distance between the information recording layers, thereby reducing crosstalk to the extent that no practical problems will be raised.
However, a multi-layered optical recording medium with three or more information recording layers has to be designed to include a number of information recording layers within a narrow region. Thus, when compared with the dual-layer structure, it is not possible to provide a more satisfactory spacing (interlayer distance) between adjacent information recording layers. Accordingly, the optical recording medium can be designed to cause crosstalk, degrading the quality of the servo signal or recorded signals. On the other hand, since no specific design techniques were available to avoid these problems, a very long period of time would be required to realize, by trial and error, an optical recording medium having three or more layers with good signal quality.